A bipolar transistor is known that includes a III-group element nitride semiconductor as a main material. FIG. 1 is a sectional view showing a typical bipolar transistor. Such a bipolar transistor is reported in “AlGaN/GaN Heterojunction Bipolar Transistor” (IEEE Electron Device Letters, Vol. 20, No. 6, pp. 277, 1999) by L. S. McCarthy, et. al.
In FIG. 1, the bipolar transistor is provided with a sapphire substrate 100, a sub collector layer 103 made of high concentration n-type GaN, a collector layer 104 made of low concentration n-type GaN, a base layer 105 made of p-type GaN, and an emitter layer 106 made of n-type Al0.1Ga0.9N. A crystal growth direction with respect to a substrate surface is parallel to a [0001] direction. An emitter electrode 10E is formed in contact with the n-type AlGaN emitter layer 106, a base electrode 10B is formed in contact with the p-type GaN base layer 105, and a collector electrode 10C is formed in contact with the n-type GaN sub collector layer 103.
As a related technique, Japanese Patent Publication JP 2004-140339A (corresponding to U.S. Pat. No. 6,856,005B2) discloses a device having a nitride hetero-structure and its manufacturing method. The device has the nitride hetero-structure made of InN itself or as a main component and has a crystal that serves as a nitrogen polarization surface or a surface having a property similar thereto on at least a portion.
Also, Japanese Patent Publication JP 2003-518776A (WO 01/048829 Gazette (corresponds to U.S. Pat. No. 6,858,509B2)) discloses a collector-up hetero-junction bipolar transistor and its manufacturing method. The hetero-junction bipolar transistor is a collector-up hetero-junction bipolar transistor. This hetero-junction bipolar transistor has an emitter layer (EM), a base layer (BA) and a collector layer (CO), which are laminated on a substrate. A surface area of a base-emitter active junction is smaller than a surface area of a base-collector active junction. A sensibility of a material of the base layer to a conductive ion implantation is lower than that of a material of the emitter layer to the same ion implantation.
Also, WO 2004/061971 Gazette (corresponds to US Patent Publication US 2005/224831A1) discloses a p-type nitride semiconductor structure and a bipolar transistor. In this p-type nitride semiconductor structure, a re-grown p-type nitride semiconductor layer is formed on a p-type nitride semiconductor subjected to etching to include In.
The inventor newly discovered the following facts. FIG. 2 is an energy band diagram of the bipolar transistor shown in FIG. 1. The energy band diagram shows a case where in the bipolar transistor shown in FIG. 1, a forward bias is applied between the base and the emitter, and a reverse bias is applied between the base and the collector. According to a band calculation, it is known that an L-M valley and a second • valley exist on a high energy side by about 2.0 eV from a bottom of a conduction band of GaN, namely, from a • valley. In FIG. 2, they are collectively shown as an “upper valley” (represented by a two-dotted chain line in FIG. 2).
As shown in FIG. 2, in the bipolar transistor in FIG. 1, electric field strength is maximum in the vicinity of a boundary between the base layer 105 and the collector layer 104. For this reason, the electrons injected from the base layer 105 into the collector layer 104 are changed to have high energy so that they receive phonon scattering to transit to the upper valley. For this reason, the bipolar transistor has a tendency that a carrier velocity decreases at a time of a high voltage operation so that a cut-off frequency is decreased. Moreover, the electrons are easy to change a high energy state in the collector layer 104, and GaN of the collector layer 104 has a relatively small band gap. Thus, avalanche breakdown is easily generated. For this reason, this bipolar transistor has a problem that a collector breakdown voltage is low.